My Wi-Fi weak security showing and net also very slow my phone name is 1phone7

This is NOT an iPhone problem; it is a problem with the settings on your Wi-Fi router.

A "Weak Security" notification means that your Wi-Fi router is not configured correctly. There is a new feature in iOS 14 and later and Mac OS Big Sur (11) and later that checks and displays a message if your router is not using the most up-to-date security settings. Note that your router has always had weak security, you just didn’t know about it until now. 

See this Apple support article for how to configure it correctly—>Recommended settings for Wi-Fi routers and access points - Apple Support, specifically the section headed Security. And don't forget to configure your range extenders (if any) also!

After you change the router settings you should Forget the network in your phone settings and reconnect.

Note also that weak security does not interfere with using Wi-Fi, it is just a warning. Your Wi-Fi should still work normally, and, if it doesn’t, it is not because of the weak security message.

I’m really happy to see that you solved the problem yourself when you marked your own post as Best Answer!

Pretty much all routers sold in the past 5 years default to the most secure settings; if your router is older than that you should consider replacing it with one that is more up-to-date.